The present invention relates to a steering system for an agricultural vehicle.
Agricultural vehicles such as combine harvesters, for example, regularly comprise a hydrostatic steering system. A conventional hydrostatic steering system is based on the principle that the steering transducer, i.e., a steering wheel, for example, is mechanically connected to a hydraulic steering valve, which, depending on the extent of actuation, feeds a certain volume of oil provided by a hydraulic pump to a steering cylinder and therefore pressurizes this steering cylinder. The deflection of the steering piston of the steering cylinder is changed in accordance with this volume of oil and depending on the direction in which the steering cylinder is pressurized, whereby the steering-knuckle steering is actuated and the steering angle of the agricultural vehicle is adjusted. Such hydraulic steering valves, which are mechanically actuated by the steering wheel and therefore have a fixed steering ratio, are defined here and in the following as the relationship between the extent of actuation of the steering transducer and the adjustment of the steering cylinder, as steering units.
Furthermore, it is known from the prior art, and especially from DE 10 2005 025 966 A1, to provide, in addition to such a steering unit, an electrohydraulic valve, which is electrically controllable by a control module and can pressurize the steering cylinder, using oil, in an identical manner either independently of the steering unit or simultaneously with the steering unit, with the result that the steering angle is changed to a corresponding extent. Such an independent control by the control module is used to implement an autopilot function such that the steering angle is therefore readjusted in accordance with a set wheel track without the steering wheel or the steering unit being actuated. By pressurizing the steering cylinder simultaneously by the steering unit and by the electrohydraulic valve, however, the steering ratio is increased and the time required for the wheels of the agricultural vehicle to reach a certain steering angle is shortened. This results from the fact that both the steering unit as well as the electrohydraulic valve pressurize the steering cylinder, possibly each using a separate pump. Such a support of the steering procedure is also referred to as “fast steering”.
It is also known from the prior art to activate the electrohydraulic valve for this type of steering support when the steering wheel is actuated either particularly rapidly, in the sense of an angular velocity, or through a particularly large angle of rotation. The reason for this is that such situations are usually an indication that a turning maneuver is supposed to be carried out with a turning circle that is as small as possible, which makes it necessary to boost the steering by means of the electrohydraulic valve. The vehicle regularly must execute a 180° turn in the headland of a field, in particular, in order to travel from one set wheel track through the crop area to the next set wheel track through the crop area to be driven along. According to another option, the operator of the steering system manually activates the electrohydraulic valve for providing the steering support when the headland area is reached, for example.
A disadvantage of the steering systems from the prior art, however, is that they are designed especially for such a turning maneuver in the headland. This relates both to its special effect on the steering behavior and to the triggering event, e.g., a maximum steering-wheel actuation for a 180° turn. Other situations in which changing the steering behavior basically makes sense are also conceivable, however, wherein this change in the steering behavior can also have requirements that differ from those for a 180° turn.